PROJECT SUMMARY Tobacco use remains the leading preventable cause of death and illness in our society. Despite this, 36.5 million US adults continue to smoke. Most of these people (70%) want to quit smoking someday, but are not yet ready to quit or actively seeking treatment. That is, they are ?ambivalent? about quitting. NCI has called for innovative, scalable, and effective mobile health (mHealth) smoking cessation interventions, but to truly reduce smoking prevalence in this country, future mHealth interventions need to effectively engage ?ambivalent smokers?, encourage them to stop smoking, and support them through this process. The current proposal is responsive to this important goal. Building on our programmatic research developing mHealth smoking cessation apps, population-level motivational interventions, and more than a year of user-centered design and development work with ambivalent smokers, we have designed a mHealth app which blends sound scientific theory, evidence-based treatment, and best practice treatment guidelines for nicotine dependence with intervention content that is also responsive to the needs and preferences of ambivalent smokers. The experimental intervention uses a series of discreet behavioral experiments (called `personal experiments') to help ambivalent smokers clarify their goals for smoking reduction/quitting; build and strengthen their motivation, confidence, and outcome expectations for quitting; and teach them the basic skills necessary to cut back and quit smoking. The experiments are paired with action-oriented guidance how to quit and a referral to evidence-based quitline counseling. Our preliminary testing found the intervention concept was well received by ambivalent smokers. They particularly liked the personal experiments, which allow them to try new behaviors and have discreet mastery experiences without having to commit to quitting smoking. This element is particularly critical. [The goals of this formative study are to complete the intervention development and evaluate the program's: a) feasibility and acceptability; b) impact on potential cognitive mediators (e.g., self-efficacy, motivation); and c) impact on indices of change in motivation or behavior (e.g., quit attempts, smoking reduction, biochemically-confirmed abstinence).] We will then plan for a future large-scale RCT to test program efficacy. The final intervention design will be informed by ambivalent smokers and experts in designing and disseminating population-level behavioral interventions for health plans, tobacco quitlines, and public health agencies. Stakeholders will also review the final results to evaluate the program's dissemination potential. The study uses a combination of qualitative and quantitative methods. Pilot participants (n=60) will be randomized to the experimental intervention or a control condition which includes the same best-practice self- help guidance and treatment referral, without the personal experiments. Participants will be followed for 3 months. If effective, this novel intervention could be disseminated through health plans, tobacco quitlines, or other public health agencies, expanding their reach to ambivalent smokers and reducing smoking prevalence.